The disclosure of Japanese Patent Application No. 2000-278681 filed on Sep. 8, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to an apparatus and a method for controlling fuel injection in an internal combustion engine and, more particularly, to a fuel injection control apparatus and a fuel injection control method for an engine having a direct fuel injection valve for injecting fuel directly into a cylinder.
2. Description of the Related Art
A widely known evaporated fuel purge apparatus prevents release of evaporated fuel (fuel vapor) from a fuel tank into the atmosphere by temporarily adsorbing fuel vapor from the tank to a canister containing activated carbon or the like and supplying (purging) fuel vapor adsorbed to the activate carbon into an engine intake passage during operation of the engine so that the fuel vapor bums in the engine.
When the purging of fuel vapor is performed, an excess amount of fuel corresponding to the amount of fuel vapor is supplied into the engine together with intake air. Therefore, if the amount of fuel injected into the engine when the purging is not performed is maintained when the purging is performed, the engine air-fuel ratio changes (decreases), so that the state of combustion in the engine may deteriorate in some cases. Therefore, according to the conventional art, when a purge is executed, the amount of fuel injected into the engine is corrected by subtracting an amount corresponding to the amount of fuel vapor supplied to the engine from the amount supplied when a purge is not executed.
An example of an engine in which the aforementioned type of reducing correction is performed is described in, for example, Japanese Patent Application Laid-Open No. 2000-27716. The engine described in this laid-open patent application is designed as a spark-ignition engine equipped with direct fuel injection valves for injecting fuel directly into the cylinders wherein, during the compression stroke of each cylinder, fuel injection is performed so that a mixture gas layer of a combustible air-fuel ratio is formed only in the vicinity of an ignition plug within the air compressed in the cylinder and containing no fuel.
According to the conventional art, the engine capable of performing the stratified charge combustion has a problem that the aforementioned purge cannot be performed during the stratified charge combustion mode. If a purge were performed to supply air containing fuel vapor into each cylinder during the stratified charge combustion mode and fuel were injected from the fuel injection valve into the fuel vapor-containing air in each cylinder during the compression stroke, the air-fuel ratio of a combustible mixture gas layer formed by the fuel injection would have a dropped air-fuel ratio. Thus, the air-fuel ratio of the combustible mixture gas layer would excessively shift to the fuel-rich side, resulting in degraded combustion. This undesired phenomenon is generally termed disturbed stratified charge combustion.
The aforementioned engine described in Japanese Patent Application Laid-Open No. 2000-27716 is designed so as to mitigate the problem of disturbed stratified charge combustion by forming swirls in each cylinder during the intake stroke and supplying fuel vapor into each cylinder through the use of swirls so that fuel vapor exists only in one of the air layer and the combustible mixture gas layer formed in each cylinder, and by correcting the amount of fuel injected in the reducing direction in accordance with the amount of fuel vapor.
However, despite the design for the localization of fuel vapor in each cylinder and the correction of reducing the amount of fuel injection in accordance with the amount of fuel vapor, the engine of the Japanese Patent Application Laid-Open No. 2000-27716 has a problem of being incapable of completely preventing disturbed stratified charge combustion.
For example, in direct fuel injection type engines, the timing of fuel injection (i.e., the fuel injection starting timing and the fuel injection ending timing) greatly affects the state of formation of a mixture gas. Therefore, the fuel injection timing is set with high precision so as to provide an optimal mixture in accordance with the amount of fuel injection, the engine revolution speed, the load, etc. Hence, if the amount of fuel injection changes, the optimal fuel injection timing changes even though the other states of engine operation, for example, the engine revolution speed, the load etc., remain unchanged. Normally, the amount of fuel injection is changed by changing the open valve duration of the fuel injection valves (injection duration). Therefore, in the engine of the aforementioned patent application as well, the fuel injection timing is changed in response to a change in the amount of fuel injection. However, normally, the fuel injection duration is controlled so that the fuel injection duration is changed by changing one of the valve opening timing of the fuel injection valves (fuel injection starting timing) and the valve closing timing of the fuel injection valves (fuel injection ending timing) while fixing the other timing. Therefore, in the aforementioned apparatus described in Japanese Patent Application Laid-Open No. 2000-27716, the fuel injection starting timing or the fuel injection ending timing is fixed even when the amount of fuel injection is reduced for correction. Thus, there is a problem of being incapable of performing fuel injection that is optimal in view of the amount of fuel injection, the engine operation state, etc.
Although the above-described problem is related to the stratified charge combustion, similar problems also occur in conjunction with an engine operation in which fuel is injected into each cylinder during the intake stroke to form a homogeneous mixture gas in the cylinder (homogeneous mixture combustion), and an engine operation in which fuel injection is performed in a divided manner during the intake stroke and during the compression stroke, and in which fuel injected during the compression stroke is stratified in a homogeneous lean mixture formed by the fuel injected during the intake stroke so that the fuel injected during the compression stroke forms a combustible mixture layer around a spark plug in each cylinder (weak stratified charge combustion). That is, similar problems occur if only one of the fuel injection starting timing and the fuel injection ending timing is changed in accordance with a change in the amount of fuel injection.
More specifically, in direct fuel injection type spark injection engines, the problem of failing to accomplish optimal combustion occurs during not only the stratified charge combustion operation but also the homogeneous mixture combustion operation and the weak stratified charge combustion operation if only the amount of fuel injection is corrected at the time of execution of a purge.
With regard to correction of the amount of fuel injection in accordance with the amount of fuel vapor as well, it is a normal practice to, during the weak stratified charge combustion operation, correct (i.e. reduce) the amount of fuel injected by the intake stroke fuel injection and the amount of fuel injected by the compression stroke fuel injection at equal rates in accordance with the amount of fuel vapor. However, since the formation of a mixture of fuel injected during the intake stroke and the formation of a mixture of fuel injected during the compression stroke are completely different, optimal mixtures cannot be formed in each cylinder if the two amounts of fuel injection are merely reduced at equal rates at the time of execution of a purge. In some cases, therefore, combustion may deteriorate.
It is an object of the invention to provide fuel injection control apparatus and method capable of controlling fuel injection so as to achieve an optimal state of combustion in accordance with operation modes (e.g., homogeneous mixture combustion, weak stratified charge combustion, stratified charge combustion, etc.) even if the purging is performed in a direct fuel injection type spark ignition engine.
A first form of the invention is a fuel injection control apparatus of a direct fuel injection type spark injection engine, including a fuel vapor purge device that supplies a fuel vapor from a fuel tank into an engine intake passage, fuel vapor detecting means for detecting an amount of the fuel vapor in an engine intake air, a direct fuel injection valve that injects fuel directly into a cylinder, and fuel injection control means for setting an amount of fuel injection from the direct fuel injection valve, a starting timing of the fuel injection and an ending timing of the fuel injection based on a state of operation of the engine. The fuel injection control apparatus further includes injection timing correcting means for changing both the starting timing and the ending timing of the fuel injection from the direct fuel injection valve of each cylinder in accordance with the amount of the fuel vapor detected by the fuel vapor detecting means.
According to the first form of the invention, the fuel vapor detecting means for detecting the amount of fuel vapor in engine intake air, so that the amount of fuel vapor in intake air can be accurately detected. The injection timing correcting means corrects the fuel injection timing so that the state of combustion in each cylinder becomes optimal, in accordance with the amount of fuel vapor detected. For example, if the amount of fuel injection (duration of fuel injection) is reduced for correction in accordance with the amount of fuel vapor, the conventional art adjusts the fuel injection duration by adjusting one of the fuel injection starting timing and the fuel injection ending timing while fixing the other timing. In this invention, however, both the fuel injection starting timing and the fuel injection ending timing are changed so that fuel injection is performed at a timing optimal with respect to the stroke (the intake stroke or the compression stroke) during which fuel injection is performed, the position of the piston, etc. More specifically, if the amount of fuel injection is to be reduced in accordance with the amount of fuel vapor, the fuel injection duration is shortened in the following manner. That is, neither the fuel injection starting timing nor the fuel injection ending timing is fixed, but both the timings are changed; for example, the fuel injection starting timing is retarded and, at the same time, the fuel injection ending timing is advanced. Therefore, the state of formation of mixture in each cylinder at the time of ignition can be optimized, and therefore the combustion in each cylinder can be optimized. Furthermore, the conventional art reduces the amount of fuel injection by the amount of fuel vapor so as to maintain a combustion air-fuel ratio regardless of the presence/absence of fuel vapor, so that the state of combustion in each cylinder becomes close to an optimal state. According to the invention, however, a more appropriate state of combustion can be achieved since the fuel injection starting timing and the fuel injection ending timing are corrected in accordance with the amount of fuel vapor. Therefore, the reduction of the amount of fuel by the amount of fuel vapor is no longer essential, and the degree of freedom in the fuel injection control increases.
In the first form of the invention, the fuel injection control means may execute fuel injection in a homogeneous combustion mode of executing the fuel injection from the fuel injection valve during an intake stroke of each cylinder so as to form a homogeneous mixture in each cylinder.
Furthermore, in the first form of the invention, the fuel injection control means may execute fuel injection in a weak stratified charge combustion mode of executing the fuel injection from the fuel injection valve during an intake stroke of each cylinder so as to form a homogeneous mixture in each cylinder, and may execute fuel injection during a compression stroke of each cylinder so as to form a mixture layer that has a low air-fuel ratio in the homogeneous mixture.
Still further, in the first form of the invention, the fuel injection control means may execute fuel injection in a stratified charge combustion mode of executing fuel injection from the fuel injection valve during a compression stroke of each cylinder so as to form a combustible mixture layer in an air in the cylinder.
Still further, in the first form of the invention, in accordance with the state of operation of the engine, the fuel injection control means may perform fuel injection by selecting one of: a homogeneous combustion mode fuel injection in which fuel injection from the fuel injection valve is performed during an intake stroke of each cylinder so as to form a homogeneous mixture in the cylinder; a weak stratified charge combustion mode fuel injection in which fuel injection from the fuel injection valve is performed during the intake stroke of each cylinder so as to form a homogeneous mixture in the cylinder, and in which fuel injection is performed during a compression stroke of each cylinder so as to form a mixture layer having a low air-fuel ratio in the homogeneous mixture; and a stratified charge combustion mode fuel injection in which fuel injection from the fuel injection valve is performed during the compression stroke of each cylinder so as to form a combustible mixture layer in an air of each cylinder.
According to the above-described forms, the correction of the fuel injection timing in accordance with the amount of fuel vapor is applied to a case where the homogeneous mixture combustion operation is performed, a case where the weak stratified charge combustion operation is performed, a case where the stratified charge combustion is performed, or a case where a suitable one of these operation modes is selected in accordance with the state of operation of the engine. That is, the fuel injection timing control is applied to any one of all the operation modes of the direct fuel injection type spark ignition engine. Therefore, it becomes possible to perform the purging of fuel vapor during any operation state of the direct fuel injection spark ignition engine. Hence, for example, even in a case where a canister-type fuel vapor purge apparatus is used, saturation of the adsorbent (e.g., activated carbon) in the canister caused by adsorption of fuel vapor is prevented, and therefore release of fuel vapor into the atmosphere is prevented.
A second form of the invention is a fuel injection control apparatus of a direct fuel injection type spark ignition engine, including a fuel vapor purge device that supplies a fuel vapor from a fuel tank into an engine intake passage, fuel vapor detecting means for detecting an amount of the fuel vapor in an engine intake air, a direct fuel injection valve that injects fuel directly into a cylinder, and fuel injection amount setting means for setting an amount of fuel injection from the direct fuel injection valve based on a state of operation of the engine. The fuel injection control apparatus further includes fuel injection control means for executing a fuel injection in which the amount of fuel injection set by the fuel injection amount setting means is injected into each cylinder in a divided manner by an intake stroke fuel injection in which fuel injection is performed during an intake stroke of each cylinder so as to form a homogeneous mixture in each cylinder, and a compression stroke fuel injection in which fuel injection is performed during a compression stroke of each cylinder so as to form a mixture layer having a low air-fuel ratio in the homogeneous mixture, and fuel injection amount correcting means for correcting an amount of fuel to be injected by the intake stroke fuel injection and an amount of fuel to be injected by the compression stroke fuel injection in accordance with the amount of the fuel vapor detected by the fuel vapor detecting means, so that an engine operation air-fuel ratio is maintained regardless of the amount of the fuel vapor, wherein the fuel injection amount correcting means reduces only the amount of fuel to be injected by the intake stroke fuel injection if the amount of the fuel vapor detected by the fuel vapor detecting means is less than the amount of fuel to be injected by the intake stroke fuel injection.
According to the second form of the invention, the fuel injection control means executes a generally-termed weak stratified charge fuel injection in which fuel injection is performed by dividing the entire amount of fuel injection for the intake stroke fuel injection and the compression stroke fuel injection, so that a combustible mixture layer is formed by the compression stroke fuel injection within the homogeneous mixture formed by the intake stroke fuel injection. Furthermore, the fuel injection amount correcting means corrects the amount of fuel injection by reducing the amount of fuel to be injected into each cylinder by an amount corresponding to the amount of fuel vapor present in engine intake air so that an engine operation air-fuel ratio is maintained regardless of the presence/absence of purged fuel vapor.
When the weak stratified charge combustion is performed, the aforementioned fuel injection amount correction is accomplished by reducing both the amount of intake stroke fuel injection and the amount of compression stroke fuel injection at equal rates so that the total of the reductions in the two amounts of fuel injection becomes equal to the amount of fuel vapor. More specifically, if the total amount of fuel injection is reduced by an amount (e.g., 10%) corresponding to the amount of fuel vapor, both the amount of intake stroke fuel injection and the amount of compression stroke fuel injection are equally reduced (by 10%) so that the total amount of fuel injection is reduced by the amount corresponding to the amount of fuel vapor.
However, the intake stroke fuel injection is performed for the purpose of forming a homogeneous mixture in each cylinder, and the compression stroke fuel injection is performed for the purpose of stratifying a relatively dense mixture in each cylinder. The fuel vapor diffuses into intake air drawn into the engine, and is supplied into each cylinder in the form of a homogeneous mixture. Therefore, if an amount of fuel vapor drawn into each cylinder in the form of a homogeneous mixture is subtracted from the amount of the compression stroke fuel injection for forming a dense mixture layer as well, the formation of a dense mixture layer may be impeded, and combustion may deteriorate in some cases. For example, if an amount of fuel vapor is subtracted from the amount of compression stroke fuel injection as well, a portion of the amount of fuel that would normally be supplied into the cylinder by the compression stroke fuel injection is replaced by an amount of fuel supplied in the form of a homogeneous mixture of fuel vapor, and therefore a fraction of the amount of fuel that should normally be stratified around the ignition plug diffuses homogeneously in the cylinder. Hence, the air-fuel ratio of a mixture layer formed by the compression stroke fuel injection shifts to a leaner air-fuel ratio, thus leading to the problem of disturbed stratified charge combustion that results in degraded combustion.
According to the invention, therefore, if any correction is to be made for the fuel vapor drawn into each cylinder in the form of a homogeneous mixture, higher priority is given to a correction made by reducing the amount of fuel injection provided by the intake stroke fuel injection for forming a homogeneous mixture. That is, in the invention, if the amount of fuel vapor drawn into the cylinder is less than the amount of fuel to be injected by the starting timing fuel injection, the correction for the amount of fuel vapor is accomplished only based on the amount of intake stroke fuel injection, and the amount of compression stroke fuel injection is not corrected. Therefore, the air-fuel ratio of homogeneous mixture that has been formed in each cylinder at the time of the compression stroke fuel injection remains the same regardless of the presence/absence of purged fuel vapor, so that disturbed stratified charge combustion will be prevented.
In the second form of the invention, the fuel injection amount correcting means may suspend the intake stroke fuel injection, and may reduce the amount of fuel to be injected by the compression stroke fuel injection, if the amount of the fuel vapor detected by the fuel vapor detecting means is greater than the amount of fuel to be injected by the intake stroke fuel injection.
Therefore, if the amount of the fuel vapor is less than the amount of fuel to be injected by the intake stroke fuel injection, the fuel injection control described above in conjunction with the second form of the invention is performed. If the amount of fuel vapor is greater than the amount of fuel to be injected by the intake stroke fuel injection, the intake stroke fuel injection is suspended, and the amount of fuel to be injected by the compression stroke fuel injection is reduced for correction so that as a whole, the amount of fuel supplied to the engine is reduced by an amount corresponding to the amount of fuel vapor. Hence, disturbance of the stratified charge combustion is minimized.
In the second form of the invention, the fuel injection amount correcting means may execute the intake stroke fuel injection by setting the amount of fuel to be injected by the intake stroke fuel injection to a predetermined amount, and may reduce the amount of fuel to be injected by the compression stroke fuel injection, if the amount of the fuel vapor detected by the fuel vapor detecting means is greater than the amount of fuel to be injected by the intake stroke fuel injection.
Therefore, if the amount of fuel vapor is less than the amount of fuel to be injected by the intake stroke fuel injection, the fuel injection control described above in conjunction with the second form of the invention is performed. If the amount of fuel vapor is greater than the amount of fuel to be injected by the intake stroke fuel injection, the intake stroke fuel injection is not suspended but is performed with a predetermined amount. The amount of fuel injection provided by the compression stroke fuel injection is reduced by the total of an amount corresponding to the amount of fuel vapor and the amount of fuel injection provided by the intake stroke fuel injection. In the weak stratified charge combustion, the fuel injected during the compression stroke forms a dense mixture layer in a lean homogeneous mixture, and flames formed by ignition in the dense mixture layer propagate to the lean homogeneous mixture. Therefore, if there is a great difference between the air-fuel ratio of the dense mixture layer and the air-fuel ratio of the homogeneous mixture, flames do not smoothly propagate from the dense mixture layer to the lean homogeneous mixture in some cases. According to the invention, even if there is a great amount of fuel vapor, the intake stroke fuel injection is not suspended, but a small amount of fuel is injected by the intake stroke fuel injection into a homogeneous mixture formed by fuel vapor, so that a mixture having a relatively low air-fuel ratio is formed in the homogeneous mixture of fuel vapor.
Therefore, the compression stroke fuel injection injects fuel into the relatively low air-fuel ratio mixture formed by the intake stroke fuel injection, so that a dense mixture layer is formed. As a result, the relatively low air-fuel ratio (intermediate air-fuel ratio) mixture formed by the intake stroke fuel injection exists between the lean homogeneous mixture formed by fuel vapor and the fuel-rich mixture layer formed by the compression stroke fuel injection. Hence, the air-fuel ratio of mixture smoothly changes from the dense mixture layer to the homogeneous mixture, so that flames smoothly propagate from the dense mixture layer to the homogeneous mixture. If in this case, the injection of a small amount of fuel during the intake stroke is performed during a latest-possible period of the intake stroke, the fuel injected by the intake stroke fuel injection does not diffuse into the homogeneous mixture, so that a mass of mixture of an intermediate air-fuel ratio can be formed in the homogeneous mixture and therefore the propagation of flames becomes smoother.